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Ivan Pavlov first demonstrated this type of adaptive anticipation, known as a conditioned response, in dogs in the 1890s. He trained the dogs to salivate in response to a stimulus by repeatedly ringing a bell before giving them food. In the microorganisms, says Pilpel, 'evolution over many generations replaces conditioned learning, but the end result is similar.' 'In both evolution and learning,' says Mitchell, 'the organism adapts its responses to environmental cues, improving its ability to survive.' Romano: 'This is not a generalized stress response, but one that is precisely geared to an anticipated event.' To see whether the microorganisms were truly exhibiting a conditioned response, Pilpel and Mitchell devised a further test for the E. coli based on another of Pavlov's experiments. When Pavlov stopped giving the dogs food after ringing the bell, the conditioned response faded until they eventually ceased salivating at its sound. The scientists did something similar, using bacteria grown by Dr. Erez Dekel, in the lab of Prof. Uri Alon of the Molecular Cell Biology Department, in an environment containing the first sugar, lactose, but not following it up with maltose. After several months, the bacteria had evolved to stop activating their maltose genes at the taste of lactose, only turning them on when maltose was actually available.

'This showed us that there is a cost to advanced preparation, but that the benefits to the organism outweigh the costs in the right circumstances,' says Pilpel. What are those circumstances? Based on the experimental evidence, the research team created a sort of cost/benefit model to predict the types of situations in which an organism could increase its chances of survival by evolving to anticipate future events. They are already planning a number of new tests for their model, as well as different avenues of experimentation based on the insights they have gained.

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